Injection molded fan blade

ABSTRACT

A variable pitch fan for a helicopter equipped with a NOTAR™ mechanical antitorque directional control system comprises a plurality of blade assemblies. Each blade assembly includes a blade portion, which is molded from a blended material which includes fiber reinforced polypropylene. A key to the invention is the use of long fibers, having a length of at least approximately one-half inch, for reinforcement, the long fibers providing for about twice the pull-out strength of short or chopped fibers. Another key aspect of the invention is the use of a blowing agent in the blended material, which serves to reduce weight and warpage by creating a foamed structure having a variable density, the structure retaining considerable strength because of the fiber reinforcement. The blade portion is molded about a spar which has holes therethrough, the holes providing a mechanical interlock between the blade and the spar.

This is a continuation, of application Ser. No. 933,115, filed Aug. 21,1992, abandoned.

BACKGROUND OF THE INVENTION

This invention relates to injection molded foamed structures, and moreparticularly to injection molded fan blades for use in the fan assemblyof a helicopter equipped with a mechanical anti-torque directionalcontrol system in which no tail rotor is employed.

A concept has been developed for countering main rotor torque inhelicopter propulsion systems by replacing the traditional tail rotorwith a constant pressure circulation control tail boom. Sideward thrustis controlled by a thruster valve located at the aft end of the tailboom. A variable pitch fan mounted at the forward end of the tail boomprovides the required airflow to the thruster. One particular version ofsuch a mechanical antitorque directional control system, denoted by thetrademark NOTAR, has been developed by McDonnell Douglas Helicopter Co.of Mesa, Arizona, and is more completely described in U.S. Pat. Nos.4,200,252 and 4,948,068, herein incorporated by reference.

The variable pitch fan for the NOTAR® control system referred to aboveincorporates a plurality of fan blades, preferably thirteen, mountedabout a fan hub having means for effecting pitch control for the blades.The blade as originally designed consists of a foam core, a fiberglassepoxy skin, and an abrasion strip bonded to the leading edge. Totalcycle time for building one composite blade is more than eight hours,which includes two hand lay-ups, two oven cures, two trims, one paintingcycle, and a secondary bonding of the abrasion strip to the bladeleading edge. Because of the labor intensive manufacturing process,fabrication costs approximate $500 per blade, with a production scraprate ranging close to 50 percent. In addition, the composite fan bladehas a brittle foam core which is susceptible to impact damage duringhandling and in operation.

Prior art injection molding processes, while capable of producing fanblades much more inexpensively than is possible using the abovedescribed composite process, would tend to produce blades havinginadequate strength characteristics and/or excess weight.

What is needed, therefore, is a fan blade, and a process for makingsame, which will be relatively inexpensive while having adequatestrength, weight, and durability characteristics to ensure dependableoperation in a crucial, demanding environment.

SUMMARY OF THE INVENTION

This invention solves the problem outlined above by providing a fanblade assembly, and a method of making same, wherein costs are reducedby up to 98 percent per blade (i.e., a cost of about $10 per blade, vs.$500 per blade using the prior art method), the production cycle time isreduced to about four minutes, the scrap rate is reduced to about twopercent, the abrasion strip and topcoat are eliminated, and impactproperties are improved.

The inventive injection molded rotor blade assembly comprises a bladeportion, molded from a blended material which includes a thermoplasticpolymer, preferably polypropylene, reinforced with long fibers, and aspar to which the blade portion is attached. A key to the invention isthe use of long fibers, having a length of at least approximatelyone-half inch, for reinforcement, the long fibers providing for abouttwice the pull-out strength of short or chopped fibers. The fiberscomprise about 15-45 percent of the blended material. Another key aspectof the invention is the use of a blowing (or foaming) agent in theblended material, preferably comprising about five percent of theblended material. The blowing agent reduces weight and warpage bycreating a foamed structure having a variable density, which retainsconsiderable strength because of the fiber reinforcement. The result isa blade portion which has optimal strength and weight characteristics.

In another aspect of the invention, a constant pressure circulationcontrol tail boom for a helicopter is disclosed. The tail boom has avariable pitch fan at its forward end for supplying a flow of airtherethrough. The fan comprises a fan hub, a plurality of pitch horns,and a plurality of fan blade assemblies, with the number of fan bladeassemblies being equal to the number of pitch horns. Each fan bladeassembly comprises a blade portion molded from a blended material whichincludes a fiber reinforced thermoplastic polymer and a blowing agent,as well as a spar to which the blade portion is attached. The spar ofeach blade is further attached to a corresponding pitch horn, so thatthe fan blade assemblies are thereby mounted to the pitch horns of thevariable pitch fan.

In yet another aspect of the invention, a method of making an injectionmolded fan blade assembly is disclosed. The method comprises the stepsof blending together a molding material which includes a fiberreinforced thermoplastic polymer, the fibers preferably being comprisedof either graphite carbon or glass, heating the molding material to aviscous molten state, injecting the molding material under high pressureinto a mold which has a spar insert in place therein about which themolding material is molded, and cooling the molding material so that itforms a blade which is attached to the spar insert to form the fan bladeassembly.

In yet another aspect of the invention, an injection molded foamedstructure having a variable density and being useful in dynamic, hightorque environments is disclosed. This structure comprises a blendedmaterial including a thermoplastic polymer reinforced with long fibershaving a length of at least approximately one-half inch and a blowingagent. The fiber reinforcement in combination with the blowing agentgives the foamed structure optimal strength and weight characteristics.This structure may be molded into a variety of parts for a number ofdifferent applications where high strength but lightweightcharacteristics are essential, such as, for example, a rotary driveshaft.

The above mentioned and other objects and features of this invention andthe manner of attaining them will become apparent, and the inventionitself will be best understood, by reference to the followingdescription taken in conjunction with the accompanying illustrativedrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side elevational view of a helicopter having a NOTARmechanical anti-torque directional control system, which includes theinventive fan blade assemblies claimed in the present application;

FIG. 2 is a perspective view, in isolation, of the fan assembly used inthe NOTAR system, including the inventive fan blade assemblies;

FIG. 3 is a top view of the spar insert which forms a part of theinventive fan blade assembly; and

FIG. 4 is a top view of a completed fan blade assembly, which shows inphantom the portion of the spar insert about which the blade portion hasbeen molded.

DETAILED DESCRIPTION OF THE DRAWINGS

Referring now to the drawings, FIG. 1 shows a helicopter 10 having atail boom 12, which incorporates a NOTAR mechanical antitorquedirectional control system developed and manufactured by McDonnellDouglas Helicopter Company, headquartered in Mesa, Arizona. The detailsof the NOTAR directional control system are taught in U.S. Pat. No.4,200,252 and 4,948,068, both owned by the same assignee as the instantapplication, and herein fully incorporated by reference, as noted in theBACKGROUND OF THE INVENTION portion of this specification.

In brief, the NOTAR system shown in FIG. 1 employs a circulation controltail boom 12 and a jet thruster 14 in place of a conventional tailrotor. In this system, low pressure air is provided by a variable pitchaxial flow fan assembly 16, which is mounted in the helicopter fuselage.This low pressure air is ejected from at least one thin horizontal slot(not shown) on the right side of the tall boom 12. The jets produced bythis air flow follow the contour of the tail boom and induce the mainrotor wake to do the same. This action produces lift on the tail boom 12in the direction required to counteract the torque produced by the mainrotor. The remaining air from the fan assembly 16 exits the tail boom 12through the jet thruster 14, which produces a force which supplementsthat produced by the circulation control tail boom. The jet thruster 14also produces all of the yaw maneuvering forces for the helicopter.

Referring now to FIG. 2, the fan assembly 16 is shown in greater detail.The fan assembly 16 includes a fan hub 18, about which a plurality ofpitch horns 20 are arrayed. Outwardly of each pitch horn 20 lies acorresponding fan blade 22. Each fan blade 22 is operatively connectedto its corresponding pitch horn 20 by means of a spar 24, which extendsthrough an aperture 26 in an outer fan ring 28. The spar 24 is in turnoperatively connected to the fan hub 18. In operation, the pitch of eachindividual fan blade 22 may be varied by means of external controls (notshown) which act through the fan hub 18 to manipulate the pitch horns20, in a manner known in the prior art.

FIGS. 3 and 4 show a fan blade assembly 30 according to the presentlyclaimed invention, which comprises both the fan blade 22 in combinationwith the spar 24. As shown particularly in FIG. 3, the spar 24 has afirst generally cylindrically shaped portion 32 and a second contouredportion 34. The second portion 34 has at least one and preferably aplurality of holes 36 therethrough, which are for a purpose to beexplained hereinbelow. FIG. 4 shows the entire fan blade assembly 30,including the spar portion 24 and the blade portion 22. The blade 22 ismolded about the second spar portion 34 in such a manner that the spar24 is firmly and securely inserted into the blade 22.

In accordance with the invention, the fan blade assembly 30 isfabricated using a one-step injection molding process. The processinvolves blending together a molding material which is primarilycomprised of a thermoplastic polymer resin which is reinforced withfibers. The preferred thermoplastic polymer is polypropylene, althoughother materials such as polyetherimide, polyphenylene sulfide,polyetheretherketone, polyphthalamide, polyamide, polysulfone,polyarylsulfone, polyethersulfone, polybutylene terephtalate,polyethylene terephthalate, or polyamide-imide, for example, may be usedif a very high temperature operating environment is contemplated. Thetradeoff when using these alternative materials would be a higher weightfactor, since polypropylene has a superior strength to weight ratio.With respect to the fibers employed in the blended material, it ispreferred to use long fibers, having a length of at least about one-halfinch, because the longer fibers provide greater surface and lineardistance of interface with the resin, and thus provide better supportstrength and mechanical properties than shorter fibers. Fan blades whichare reinforced with long fibers, rather than short fibers, have beenshown in tests conducted by the inventor to have approximately twice thepull-out strength. Preferably, the fibers should comprise about 15-45percent of the entire material mix, with that range being optimized atabout 30 percent. It is preferred that the fibers be comprised of eitherglass or graphite carbon, though other fiber materials could beemployed.

In addition to the 30 percent long fiber reinforced polypropylene, theblended material preferably includes both a blowing (foaming) agent, anda color pigment (preferably white), as well as a UV stabilizer. Theblowing agent is preferably celogen, and should comprise about 1-10percent of the blended material, with 5 percent being preferred. Thecolor pigment preferably comprises about one-half to one percent of thematerial. The blowing agent, which makes the polypropylene open-celled,is used to reduce weight and warpage, and in fact reduces the weight ofthe blade assembly 30 by as much as 20 percent. The quantity of blowingagent employed is critical, however, since the blowing agent tends toundermine the reinforcement provided by the long glass or graphitecarbon fibers, requiring a careful balance to provide optimumproperties.

Once the molding material has been blended, the second step in theinventive process is to heat the material to a viscous molten state.Then, the molten molding material is injected under high pressure into amold where the spar insert 24 has previously been placed to be moldedaround. Once the injection step is completed the blade assembly isessentially finished. The mold is cooled in a known fashion, and thenthe finished blade assembly 30 is removed therefrom.

As noted above, the spar 24 functions as the backbone for the bladeassembly 30, and provides a means of attachment to the pitch controlhorn 20 and fan hub 18 (FIG. 2). The holes 36 in the second spar portion34 become filled with the blended material which forms the blade portion22 during the injection step, and thereby serve as mechanical fastenersto interlock with the blade 22, thus assuring that blade 22 and the spar24 are securely attached.

Although injection molding has been widely used in both the automotiveand aerospace industries, the fan blade as described herein is the firstof its kind to be used as a critical structure on an aircraft. Thismaterial application fully utilizes the unique characteristics of afoamed plastic part-the formation of a solid skin on the part surface incontact with the walls of the mold and the insert. The combination ofthis solid skin, the long fibers, and the blowing agent producestiffness, weight, load strength and durability properties desired forapplications such as the high shear, high speed NOTAR system fanstructure.

Although an exemplary embodiment of the invention has been shown anddescribed, many changes, modifications, and substitutions may be made byone having ordinary skill in the art without departing from the spiritand scope of the invention. In particular, it should be noted that theinvention has much broader applicability than as a fan blade in a NOTARantitorque control system, since such a fiber reinforced, variabledensity, injection molded blade may be used in many different rotorblade applications. The invention could even be applied to otherapplications unrelated to the rotor blade field, particularly indynamic, high torque environments, where strength and weight arecritical parameters. An example of such an environment would be toemploy the invention as a rotary drive shaft or the like. Therefore, thescope of the invention is to be limited only in accordance with thefollowing claims.

What is claimed is:
 1. An injection molded rotor blade assembly,comprising:a blade portion molded from a blended material which includesa thermoplastic polymer reinforced with long fibers and a blowing agent,wherein said blade portion comprises an interior portion having agenerally open-celled structure, said interior portion being enclosed byan exterior skin formed from the blended material, which issubstantially solid; and a spar to which said blade portion is attached.2. The rotor blade assembly as recited in claim 1, wherein said longfibers have a length of at least approximately one-half inch.
 3. Therotor blade assembly as recited in claim 1, wherein said long fiberscomprise approximately 15-45 percent of said blended material.
 4. Therotor blade assembly as recited in claim 1, wherein said long fiberscomprise approximately 30 percent of said blended material.
 5. The rotorblade assembly as recited in claim 1, wherein said thermoplastic polymercomprises polypropylene.
 6. The rotor blade assembly as recited in claim1, wherein said fibers are comprised of either glass or graphite carbon.7. The rotor blade assembly as recited in claim 1, wherein said blowingagent comprises approximately 1-10 percent of said blended material. 8.The rotor blade assembly as recited in claim 7, wherein said blowingagent comprises approximately 5 percent of said blended material.
 9. Therotor blade assembly as recited in claim 1, wherein said blendedmaterial further comprises a color pigment, said color pigmentcomprising approximately 0.5-1 percent of the blended material.
 10. Therotor blade assembly as recited in claim 1, wherein said spar includesone or more holes therethrough, said blade portion being molded aboutsaid spar, such that said blade portion blended material fills saidholes, thereby mechanically locking said blade portion to said spar. 11.An injection molded rotor blade assembly, comprising:a blade portionmolded from a blended material, said blended material includingpolypropylene reinforced with fibers and a blowing agent, wherein saidblade portion comprises an interior portion having a generally celledstructure, said interior portion being enclosed by an exterior skinformed from the blended material, which is substantially solid; and aspar to which said blade portion is attached.
 12. The rotor bladeassembly as recited in claim 11, wherein said fibers are long fibershaving a length of at least approximately one-half inch, and arecomprised of either glass or graphite carbon.
 13. The rotor bladeassembly as recited in claim 12, wherein said long fibers compriseapproximately 15-45 percent of said blended material.
 14. The rotorblade assembly as recited in claim 12, wherein said long fibers compriseapproximately 30 percent of said blended material.
 15. The rotor bladeassembly as recited in claim 11, wherein said blowing agent comprisesapproximately 1-10 percent of said blended material.
 16. The rotor bladeassembly as recited in claim 11, wherein said blowing agent comprisesapproximately 5 percent of said blended material.
 17. The rotor bladeassembly as recited in claim 11, wherein said spar includes one or moreholes therethrough, said blade portion being molded about said spar,such that said blade portion blended material fills said holes, therebymechanically locking said blade portion to said spar.
 18. A constantpressure circulation control tail boom for a helicopter, said tail boomhaving a variable pitch fan at its forward end for supplying a flow ofair therethrough, said fan comprising:a fan hub; a plurality of pitchhorns; and a plurality of fan blade assemblies, the number of fan bladeassemblies being equal to the number of pitch horns, each said fan bladeassembly comprising: a blade portion molded from a blended materialwhich includes a thermoplastic polymer reinforced with long fibers and ablowing agent, wherein said blade portion comprises an interior portionhaving a generally open-celled structure, said interior portion beingenclosed by an exterior skin formed from the blended material, which issubstantially solid; and a spar to which said blade portion is attached;wherein the spar of each blade assembly is further attached to acorresponding pitch horn, such that the fan blade assemblies are therebymounted to the pitch horns of the variable pitch fan.
 19. Thecirculation control boom as recited in claim 18, wherein said fibershave a length of at least approximately one-half inch and are comprisedof either glass or graphite carbon.
 20. The circulation control tailboom as recited in claim 19, wherein said long fibers compriseapproximately 15-45 percent of said blended material.
 21. Thecirculation control tail boom as recited in claim 19, wherein said longfibers comprise approximately 30 percent of said blended material. 22.The circulation control tail boom as recited in claim 18, wherein saidblowing agent comprises approximately 1-10 percent of said blendedmaterial.
 23. The circulation control tail boom as recited in claim 18,wherein said blowing agent comprises approximately 5 percent of saidblended material.
 24. The circulation control tail boom as recited inclaim 18, wherein said spar includes one or more holes therethrough,said blade portion being molded about said spar, such that said bladeportion blended material fills said holes, thereby mechanically lockingsaid blade portion to said spar.
 25. The circulation control boom asrecited in claim 18, wherein there are thirteen fan blade assemblies insaid variable pitch fan.